[setr]

Theres only ~300 million people in the us, so the gp's source is claiming something like 10% poverty

Something's clearly misaligned with the definition of poverty between your sources; you're differing by an order of magnitude

[schoen]

That's a huge difference. I assume different sources did use quite different measures:

https://en.wikipedia.org/wiki/Poverty#Measuring_poverty

https://en.wikipedia.org/wiki/Poverty_threshold